Such a boiler comprises, inter alia, the following elements:
a circulating fluidized bed hearth whose walls comprise vertical pipes in which an emulsion (liquid phase and vapor phase) of water circulates, which emulsion is produced by evaporation of water fed into the bases of said pipes; PA1 a separator member (in general a cyclone) which, at the top of the hearth, recovers the flue gas and the solid particles fluidized in the hearth and directs said flue gas and said particles to respective ones of two distinct ducts; and PA1 an external fluidized bed connected firstly to the particles duct of the separator member and secondly to the base of the hearth, the external bed including a heat exchanger which transmits the heat from the particles to the coolant fluid to be evaporated, which fluid is often water.
Such a heat exchanger is commonly in the form of a nest of tubes and the fluid can circulate either naturally or with assistance from one or more pumps.
Natural-circulation heat exchangers are difficult to implement. Firstly, in order to operate at reasonable circulation rates, the tubes must be disposed so that they slope, which makes the heat exchanger taller than a heat exchanger in which the tubes are horizontal, if substantially the same heat exchange area is to be retained. Unfortunately, the height of the fluidized bed is limited by the fluidization pressure, and also by the fluidization stability. It can be assumed that said height cannot significantly exceed 4 meters, and, in any case, it would seem impossible to provide a height greater than 5 meters. Secondly, the speed in the tubes must be suitable for avoiding "departure from nucleate boiling" (DNB), and also for avoiding stratified flow, i.e. the appearance of a liquid-vapor interface. A flow of that type gives rise to overheating of the tubes, to thermal shocks, and to corrosion. To achieve the required speed, the tubes must be of large section and have large clearance heights at their top ends. In addition, during start-up, there is not a large enough quantity of hot particles present, so that, since vaporization is low, the speed in the tubes is too low and there is a risk of DNB.
Assisted-circulation heat exchangers make it possible to overcome the difficulties encountered with natural-circulation heat exchangers. Unfortunately, assistance pumps are costly, and there is a risk that they might break down, which is detrimental to boiler availability.